Recombinant microorganisms have widespread utility and importance. One use of these microorganisms is as live vaccines to produce an immune response. When the recombinant microorganism is used as a vertebrate live vaccine, certain considerations must be taken into account. To provide a benefit beyond that of a nonliving vaccine, the live vaccine microorganism must attach to, invade, and survive in lymphoid tissues of the vertebrate and expose these immune effector sites to antigen for an extended period of time. Through this continual stimulation, the vertebrate's immune system becomes more highly reactive to the antigen than with a nonliving vaccine. Therefore, preferred live vaccines are attenuated pathogens of the vertebrate, particularly pathogens that colonize the gut-associated lymphoid tissue (GALT), nasopharynx-associated lymphoid tissue (NALT) or bronchial-associated lymphoid tissue (BALT). An additional advantage of these attenuated pathogens over nonliving vaccines is that these pathogens have elaborate mechanisms to gain access to lymphoid tissues, and thus efficient exposure to the vertebrate's immune system can be expected. In contrast, nonliving vaccines will only provide an immune stimulus if the vaccine is passively exposed to the immune system, or if host mechanisms bring the vaccine to the immune system.
Appropriate attenuation and biocontainment of a live recombinant vaccine is essential. Additionally, a live recombinant vaccine should be capable of delivering antigen to the cytosol of a host cell, if necessary for the appropriate immune response (i.e. cellular vs. humoral). Consequently, there is a need in the art for a recombinant bacterium that may be used as a live vaccine that is attenuated, capable of biocontainment, capable of expressing and/or synthesizing antigen, and capable of delivering antigen or a nucleic acid encoding an antigen to a host cell.